Plating on aluminum



United States Patent 3,193,474 PLATKNG 0N ALUMINUM Peter G. Kenedi, Detroit, Mich, and Robert T. Gore, Middletown, NJ, assignors', by mesne assignments, to Mt zT Chemicals inc, New York, N.Y., a corporation of Deiaware N 0 Drawing. Original appiication July 13, 1959, Ser. No. 826,497, new Patent No. 3,108,006, dated Oct. 22, 1963. Divided and this application Aug. 27, 1962, Ser. No.

16 laims. (Cl. 204-38) This is a divisional application of our copending application Serial No. 826,497, filed July 13, 1959 which has issued as US. Patent No. 3,108,006 on October 22, 1963.

The present invention relates to plating on aluminum.

Aluminum, which is abundant, economical, and has favorable physical and electrical properties, is used in large quantities. Its use in certain fields has been limited by its surface characteristics. Aluminum surfaces oxidize to form a tight adherent refractory film which interferes with surface-to-surface contact. Aluminum surfaces cannot be joined by soldering.

Techniques have been developed for both immersion plating and electroplating on aluminum. One of the few useful, although limited, techniques is the immersion tinning of certain aluminum alloys using alkali stannate solutions. This technique is successful only when applied to aluminum alloys containing several percent of copper.

The most widely used process for plating on aluminum is that in which an immersion zinc coating replaces the oxide film. To be successfully applied, immersion zinc processes require close control of a large number of process steps and variables. This has limited its industrial utilization and applicability. A need exists for a relatively simple and widely applicable method for plating on aluminum.

It is an object of the present invention to provide a bath and a process for immersion tinning aluminum.

It is a further object of the present invention to provide a process for electrotinning aluminum.

The invention also contemplates providing a bath useful without change for both immersion plating and electroplating tin on aluminum.

It is still another object of the present invention to provide a process for electroplating one or more metals on aluminum for decorative and/or industrial purposes.

The invention also contemplates a continuous method of plating aluminum wire and sheet.

We discovered that when aluminum, having a properly cleaned surface, is immersed in an aqueous stannate-cyanide bath having the following preferred composition; CuCN-about 11 g./l. (grams per liter), free KCN- about 26 g./l., KOH-about 7.5 g./l., and K Sn(OH) about 120 g./l., at a temperature of about 50 C., an adherent tin layer is chemically deposited on the aluminum surface in about to 30 seconds. The aluminum may be retained in the immersion bath for as long as 2 to 3 minutes at which time further deposition practically ceases. A deposit of about 0.00025 cm. is built up, dependent somewhat upon the temperature and specific bath utilized. Thick immersion deposits of this type are somewhat porous but have sufficient continuity and stability to (i) protect the aluminum surface from oxidation; (ii) provide a surface suitable to making sliding contact; and (iii) provide an aluminum article with suflicient conductivity at the surface during its service life to enable the part to be used as an electric conductor, e.g., for screw or friction type electric contacts. The adhesion of these immersion de- 3,193,474 Patented July 6, 1965 "ice posits is markedly improved by reflowing the deposit at an elevated temperature.

An important feature of this process is the successful immersion tin coating formed on the aluminum surface in the stannate-cyanide solution. When the aluminum is immersed in the solution, a dark flash deposit takes place within about 1 to 3 seconds and continues for about 2 seconds, followed almost instantaneously by a white deposition. As the stannate-cyanide solution is transparent and fairly clear, the initial dark flash followed by the deposition of a white metal is easily observed. This initial flash is believed to be an extremely thin layer of copper. The subsequent white deposit is tin. Analyses of thick immersion deposits (2 to 3 minutes immersion) have shown them to have a tin content greater than 99%. The immersion deposit which is believed to contain a small amount of copper in the tin alloy is referred to herein as an immersion tin deposit.

Although thick immersion deposits obtained by long immersion have many uses, they are not suitable as an undercoat for adherent electrodeposits. To obtain sound adherent electrodeposits, the undercoat should be one obtained by immersion for not more than about 30 or 40 seconds.

We also discovered that a sound and adherent tincopper alloy plate may be electrodeposited on aluminum surfaced with a thin immersion tin deposit prepared by immersing the aluminum in a bath of thetype specified herein for between approximately 5 and not more than 40 seconds, and preferably not more than 15 seconds. The electrodeposition of the tin-copper alloy is preferably carried out in a bath of the same composition as that useful for the immersion dip. The immersion dip and the subsequent electrodeposition may be carried out in the same tank or in separate tanks. Aluminum surfaced with this tin-copper alloy is suitable for many purposes. The alloys plated from baths also useful for the immersion dip, contain between 40% and 60% of tin, the remainder copper. Aluminum surfaced as aforesaid is also useful as a basis metal for one or more subsequent electrodeposits to provide decorative and/or utilitarian finishes. Without further finishing it is useful for a wide variety of applications, including tin plated wire, tin coated sheet, fabrication of cans, electrical applications, sliding and other bearing-type applications, etc.

We have also discovered that other metals, such as copper, tin, bronze, nickel, etc., may be deposited directly on the thin tin immersion coating. The thin immersion tin deposit on the aluminum is susceptible to corrosive attack by the plating solution utilized for electrodepositing the subsequent layer(s). Baths and plating conditions must be selected wherein the'electrodeposition process proceeds at such a rate and under such conditions that electrodeposition is preferential to the rate of corrosive attack. Generally the more neutral solutions are those from which plating is successfully accomplished; the preferred pH range being between about 4 and about 9. Such metals as copper and bronze have been plated from pyrophosphate solutions, and bright nickel from a modified Watts solution. Although it is possible and often desirable to electrodeposit metals other than the copper-tin alloy directly on the thin immersion deposit, it is generally preferred that a relatively thin tin-copper alloy be electrodeposited on the thin immersion deposit before electrodepositing the desired metal. Various metals, including bronze, tin-nickel, copper, and chromium have been successfully electrodeposited using a tin-copper electrodeposit as the undercoat.

The immersion bath specified hereinbefore is a composition which has been found particularly suitable for both immersion and electroplating. Adherent immersion tin deposits may be obtained from aqueous baths 1 relative propensity of copper and tin to dcposit from solution. The relative propensity of copper to deposit from solution is increased with high concentrations of copper cyanide and caustic, and with low concentration of the type: 5 of free cyanide and stannate. Low temperatures also 7 promote copper deposition. By proper manipulation of Broa Farmed these factors it is possible to obtain sound deposits outi fi 3-1 side the range specified hereinbefore. It is sometimes possible to obtain immersion deposits in stannate-cyanide Smqadded as KSMOHM 2H9 35551. 10 baths of the type specified hereinbefore with a dissolved Cu+[added as GuON]--- 3 5-11.5 5.7l0.6 copper content as low as about 1.5 g./l., by proper 3 p {g5}; 238 manipulation of these various factors and by the addition to the solution of such materials as tartrates, citrates,

In addition to the materials noted in the table, the aquegluconates and i i whlch are known to ous baths contained sufiicient additional KCN to com- Heine the deposmon of copper i i bronze electro- 'P1ex the copper [cu(CN)3=] Thus, the Solution com plating baths. The use of these add1t1ves also sharply tains about 3 mols of CN for each mol of Cu' in adlowers the i amount of cppper m operiiiwe dition to the free cyanide. This method of specifying baths opleratwe baths e a sniau cntlcal the solution composition is common for cyanide copper gmqunt. of dlsso copper efiecnve to obtam adherent solutions and is used herein. i g

It is possible to obtain immersion tin deposits from electrodeposltloniof tm'copper Preferably stannate solutions outside the limits specified hereinbeearned q baflispf t Same composllltlon. as those fore. For aluminum alloys, .otherthan the'high copper usled t e Immersion i t tm'iiopper alloys, deposits from sodium stannate/baths are not ada1 1S electmdepwte! lmmerslon tm herent Those deposits Prepared from baths within the posit. The electrodeposltion is earned out between 32 preferred range and those haying a copper to tin ratio and 659 9 Preferably between 48 55 i h l i between 1;4 5 and 1: 5 are the most Current densities conventlonal for copper-tin alloy platadherent. The preferred operating range is about 48 C. i are Suitable These en y y from 1 p .to 55 (1., although it i possible in the preferred b th 3O dm. to 15 amp/sq. dm. for conventional batch plating composition range specified to operate between 32 C. a y be g as 150 mp qg for and 65 C. The temperature range for obtaining ad.- eoiltinuolis i an Strip Plating The pp alloy herent deposits is somewhat narrower for'a-luminum almay be l r p i to a y desired ihieklless- Its loys'having high silicon and/or magnesium content. app is best at a thickness of 0-0025 t0 0-00025 Baths prepared from the salts hereinbefore specified 35 cm. It is preferable that the electrodeposited coating are preferred. It is possible to obtain adherent immerbe at least 0.001 cm. thick if it is to serve as theundersion tin deposits from baths in which the sodium salts coat for 'electrodeposition Of other metals deposited from replace in part the potassium salts. For a given bath 1 g y Caustic Solutions, if it is to be used in pp composition more adherent deposits are obtained from tions requiring soldering to produce joints with good the all-potassium bath. Deposits become progressively 40 physical properties. ,The deposit may be brightened by less adherent s sodium replaces p a m- Bat s' n the addition ofa lead-containing brightener to the platfainiiig less h 313011? 2 Potassium ions Sodium ing solution. A multi-layered deposit with alternate tin 9 result d p having e e mfel'loi' mike" and tin-copper layers is obtained by interruption of the i characienstlcs' w replafimg some of the-Poms current during electrodeposition using a bath specified slum salts by the equivalent sodium salts, the werght f herein g g j g i g if ga fi zg 31 53 5 i 2 155 1: For the purpose of giving those skilled in the art a Bible to fy the Salts somewhat by add'ing both 605p better understand ng of the invention, the following illusand some of the tin in the form of copper stannate, if examp P are gwen: I suflicient cyanide is present to form the copper complex A .Fommerclany Pure Sheet. a ummum" 1100 (25) and also to provide suflicient free cyanide within the for F Y Purposes' Before f 'range specified. Copper may also be added in the form nferslon e sPeclfifid 9 was cleaned f of a Potassium copper cyanide ping 111 a caustic solution contalnlng 30 g./l. of tr1sod1um The amount of dissolved copper in the solution is Phosphate and 22 8 soda betweeii a critical factor involved in obtaining sound immersion 55 and It Was then rinsed lncold PP in deposits. Although it is generally impossible to obtain a eoneenliiaied mine aeld soiutleii and 5 {insertisound deposits with a copper concentration outside the The seluilons tested and the eondltlons p i in the limits specified herein, it is sometimes possible to do so following table resulted in sound. adherent depeslis- All by careful selection of the other bath components and t e electrcdeposits were made on thin immersion'deby the use of special addition agents which affect the GO "posits.

Example OuCN, Free KOH, K Sn(OH) Type of Temp, Additive, Number g/l. N, g./l. g./1 Deposition C. g./l.

16 22.5 7.5 Immersion 58 16 22.5 7.5 60 Electro 54 10.5 22.5 7.5 120 Immersiom- 755 10.5 22.5 7.5 120 Electro 58 16 22.5 7.5 120 Inmersion. 58 16 22.5 7.5 120 Electra 68 16 22.5 7.5 Immersion- 55 16 22.5 7.5 90 Electro 60 4.8 22.5 7.5 Immersiom- 49 Citric acid- 7.5 22.5 7.5 120 Electro 55 oizt'r ie acid- In addition to the deposition from the stannate-cyanide also plated by electrodepositing nickel directly on the thin immersion tin deposit from a standard Watts nickel solution under standard conditions. In a similar manner, copper was electrodeposited from a pyrophosphate copper solution.

Industrially useful and/ or decorative finishes of a composite nature have also been successfully electrodeposited on aluminum. The surface was first coated with a thin adherent immersion deposit, followed by an electrodeposit from the same bath. On this base, (i) bronze has been electrodeposited from a cyanide bath, (ii) copper from a pyrophosphate copper bath, and (iii)' coppernickel-chromium from conventional baths.

Aluminum wire was surfaced with the tin-copper electrodeposit from the same bath used to obtain the immersion tin undercoat. An extremely high current density was used. This process is particularly suitable to the electrocoating of wire and sheet in continuous high speed mills.

The process has been successfully applied to a wide range of aluminum alloys. These include 1100 (2S), 3003 (3S), 2017 (178), 2024 (248), 5052 (525), 6061 (618), 7075 (758), EC grade wire, and Al-356 (sandcast). Before applying immersion deposits the aluminum surface must be cleaned. Cleaning steps usually involve initial solvent cleaning, followed by an alkaline dip, a water rinse, and an acid dip. The best procedure varies for different alloys. These are conventional and specified in ASTM B253-53, as well as Metal Finishing Guidebook Directory (1959).

As many embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention includes all such modifications and variations as come within the scope of the appended claims.

We claim:

1. A process for electrodepositing a tin-copper alloy on aluminum comprising immersing a piece of cleaned aluminum, for between about 5 seconds and 40 seconds, in an aqueous solution containing about 23 g./l. to 59 g./l. of Su 15 g./l. to 38 g./l. of free alkali metal cyanide, 3 g./l. to 12 g./l. of alkali metal hydroxide, at least 20% of said alkali metal in the solution being potassium and the remainder sodium, and sufficient Cu+ selected from the range of 1.5 g./l. to 11.5 g./l. to form an adherent immersion tin deposit; and then electrodepositing a tincopper alloy on the initial immersion deposit from a bath having a composition within the range specified for the immersion bath.

2. A process for electrodepositing a tin-copper alloy on aluminum comprising immersing a piece of cleaned aluminum, for between about 5 seconds and 15 seconds, in an aqueous solution containing about 23 g./l. to 59 g./l. of Sn, 15 g./l. to 38 g./l. of free alkali metal cyanide, 3 g./l. to 12 g./l. of alkali metal hydroxide, at least 20% of said alkali metal solution being potassium and the remainder sodium, and about 3.5 g./l. to 11.5 g./l. of Cu+, the ratio of copper to tin being about 1:45 and 1:65, at a temperature of about 48 C. and 55 C., to form a thin immersion tin deposit on the aluminum; and then electrodepositing a tin-copper alloy on the initial immersion deposit from a bath having a composition within the range specified for the immersion bath, at a temperature between about 32 C. and 65 C.

3. A process for electrodepositing a tin-copper alloy on aluminum comprising immersing a piece of cleaned aluminum for between about 5 seconds and 15 seconds, in an aqueous solution containing about 35.6 g./l. to 51.5 g./l. of Sn+ 22 g./l. to 30 g./l. of free potassium cyanide, 5 g./l. to g./l. of potassium hydroxide, and about 5.7 g./1. to 10.6 g./l. of Cu+, at a temperature of about 32 C. and 65 C., to form a thin immersion tin deposit on the aluminum; and then electrodepositing a tin-copper alloy on the initial immersion deposit from a bath having a composition within the range specified for the immersion bath, at a temperature between about 32 C. and C.

4. A process for electrodepositing a tin-copper alloy on aluminum comprising immersing a piece of cleaned aluminum for between about 5 seconds and 30 seconds, in an aqueous solution containing about 35.6 g./l. to 51.5 g./l. of Sn, 22 g./l. to 30' g./l. of free alkali metal cyanide, 5 g./l. to 10,g./l. of alkali metal hydroxide, at least 20% of said alkali metal in the solution being potassium and the remainder sodium, and about 5.7 g./l. to 10.6 g./1. of Cu' at a temperature of about 48 C. and 55 C., to form a thin immersion tin deposit on the aluminum, and then electrodepositing a tin-copper alloy on the initial immersion deposit from a bath having a composition within the range specified for the immersion bath, at a temperature between about 48 C. and 55 C.

5. A process for electrodepositing a tin-copper alloy on aluminum comprising immersing a piece of cleaned aluminum, for between about 5 seconds and 15 seconds, in an aqueous solution containing about g./l. to g./l. of potassium stannate, 22 g./l. i030 g./l. of free potassium cyanide, 5 g./l. to 10 g./l. of potassium hydroxide, and 8 g./l. to 16 g./l. of copper cyanide, at a temperature of about 48 C. and 55 C., to form a thin immersion tin deposit on the aluminum; and then electrodepositing a tin-copper alloy on the initial immersion deposit from a bath having a composition within the range specified for the immersion bath, at a temperature between about 48 C. and 55 C.

6. A process'for electrodepositing a metal on aluminum comprising immersing a piece of cleaned aluminum, for between about 5 seconds and 40 seconds, in an aqueous solution containing about 23 g./l. to 59 g./l. of Sn, 15 g./l. to 38 g./l. of free alkali metal cyanide, 3 g./l. to 12 g./l. of alkali metal hydroxide, at least 20% of said alkali metal in the solution beingpotassium and the remainder sodium, and sufiicient Cu+ selected from the range of 1.5 g./l. to 11.5 g./l. to form an adherent immersion tin deposit; and then electrodepositing a metal on the initial immersion deposit.

7. The process of claim 6 in which the metal is electrodeposited from a bath having a pH between about 4 and 9.

8. A process for electrodepositing a metal on aluminum comprising immersing a piece of cleaned aluminum, for between about 5 seconds and 15 seconds, in an aque ous solution containing about 35.6 g./l. to 51.5 g./l. of Sn+ 22 g./l. to 30 g./l. of free potassium cyanide, 5 g./l. to 10 g./l. of potassium hydroxide, and about 5.7 g./l. to 10.6 g./l. of Cu+, at a temperature between about 32 C. and 65 C. to produce an immersion tin deposit containing at least 99% tin on said aluminum; and then electrodepositing a metal on the initial immersion deposit.

9. The process of claim 8 in which the metal is electrodeposited from a bath having a pH between about 4 and 9.

10. A process for producing a layered deposit on aluminum comprising immersing cleaned aluminum in an aqueous solution containing about 23 g./l. to 59 g./l. of Sn, 15 g./l. to 38 g./l. of free alkali metal cyanide, 3 g./l. to 12 g./l. of alkali metal hydroxide, at least 20% of said alkali metal in the solution being potassium and the remainder sodium, and sufiicient Cu+ selected from the range of 1.5 g./l. to 11.5 g./l. to form an adherent immersion tin deposit, for between about 5 seconds and 40 seconds; and then electrodepositing a tincopper alloy on the initial immersion deposit from the same bath; and then, alternately, deposit immersion tin and electrolytic tin-copper by interrupting the current for between 5 seconds and 40 seconds for each immersion deposit.

11. A process for a producing a layered deposit on aluminum comprising immersing cleaned aluminum in an aqueous solution containing about.35.6 g./.l. to 51.5

g./l. of Sn, 22 g./1. to 30 g./l. of free potassium cyanide, g./l. to 10 g./l. of potassium hydroxide, and 5.7 g./l. to 10.6 g./l. of.Cu+, to form an adherent immersion tin deposit, for between about 5 second and .15 seconds; and then electrodepositing a tin-copper 'alloy on the initial immersion deposit from the same bath; and then alternately, deposit, immersion tin and electrolytic tin-copper for interrupting the current for between 5 seconds and seconds for each immersion deposit.

12. A process for electroplating aluminum wire and sheet comprising passing a continuous. metal strip of cleaned aluminum through an immersion solution containing about 23 g./1. to 59 g./l. of S11, 15 g./l. to 38 g./l. of free alkali metal cyanide,'3 g.'/l. to 12 g./l. of alkali metal hydroxide, at least of said alkali metal in the solution being potassium and the remainder sodium, and sufiicient Cu+ selected from the range of 1.5 g./l. to 11.5 g./l. to form an adherent immersion tin deposit, for between about 5 seconds and 40 seconds; and then passing the coated aluminum, as a cathode, through a bath having a compositionrwithin the range specified for the immersion bath to electrodeposit a tincopper alloy on the initial immersion deposit. 13. A process for electroplating aluminum wire and sheet comprising passing a continuous metal strip of cleaned aluminum through an immersion solution containing about 35.6 g./l. to 51.5 g./l. of Sn, 22 g./l. to g./l. of free potassium cyanide, 5 g./l. to 10 g./l. of potassium hydroxide,'and 5.7 g./l. to 10.6 g./l. of Cu+, to form an adherent immersion tin deposit, for between about 5 seconds and 15 seconds; and then passing the coated aluminum, as a cathode, through a bath having a composition within the range specified for the immersion bath to electrodeposit a tin-copper alloy on the initial immersion deposit.

14. A process for electroplating aluminum wire and sheet comprising passing a continuous metal. stripof cleaned aluminum through .an immersion solution. containing about g./l. to' l30 g. /l. of potassium stannate, 22 g. /l. to 30 g./l. of free potassium cyanide, 5 g./l. to 10 g./ l. potassium hydroxide, and 8 g./1. to 16 g./l. of copper cyanide, to from an adherent immersion tin deposit, for between about 5 seconds and 15 seconds; and then passing the coated aluminum, as a cathode, through a'bath having a composition within the range specified for the immersion bath to electrodeposit a tincopper alloy on the initial immersion deposit.

15. A process for electroplating aluminum wire and sheet comprising passing a continuous metal strip of cleaned aluminum through an immersion solution con taining about 35.6 g./l. to 5 1.5 g./l. of Sn, 22 'g./l. to

30 g./l. of free potassium cyanide, 5 g./l. to 10 g./l. of

potassium hydroxide, and 5.7 g./l. to 10.6 g./l. of Cu to form an adherent immersion tin deposit, for between about 5 seconds and 30 seconds; and 7 then passing the coated aluminum, as a cathode, through a metal plating bath and electrodepositing a metal on the initial im- References Cited by the Examiner UNITED STATES PATENTS Metal Industry, 83:461-3, 502-4 (1953).

WINSTON A. DOUGLAS, Primary Examiner. MURRAY TILLMAN, JOHN H. MACK, Examiners. 

6. A PROCESS FOR ELECTRODEPOSITING A METAL ON ALUMINUM COMPRISING IMMERSING A PIECE OF CLEANED ALUMINUM, FOR BETWEEN ABOUT 5 SECONDS AND 40 SECONDS, IN AN AQUEOUS SOLUTION CONTAINING ABOUT 23 G./L. TO 59 G./L. OF SN+4, 15 G./L. TO 38 G./L. OF FREE ALKALI METAL CYAMIDE, 3 G./L. TO 12 G./L. OF ALKALI METAL HYDROXIDE, AT LEAST 20% OF SAID ALKALI METAL IN THE SOLUTION BEING POTASSIUM AND THE REMAINDER SODIUM, AND SUFFICIENT CU+ SELECTED FROM THE RANGE OF 1.5 G./L. TO 11.5 G./L. TO FORM AN ADHERENT IMMERSION TIN DEPOSIT; AND THEN ELECTRODEPOSITING A METAL ON THE INITIAL IMMERSION DEPOSIT. 